Syntheses and properties of electroluminescent polyfluorene-based conjugated polymers, containing oxadiazole and carbazole units as pendants, for LEDs

New polyfluorenes (PF)-based conjugated copolymers, containing oxadiazole and carbazole units as pendants, were prepared as the electroluminescent (EL) layer in light-emitting diodes (LEDs) to show that most of them have higher maximum brightness and EL efficiency as compared to poly(2,7-(9,9-bis(2-ethylhexyl)fluorene)) (PF2/6). The prepared polymers, poly[(9-(6-(N-carbazolyl)-hexyl)-9-hexyl)-fluorene-2,7-diyl]-co-[(9-hexyl-9-(6-(4-(5-phenyl-1,3,4-oxadiazolyl)-phenoxy)-hexyl)-fluorene-2,7-diyl)] (Oxd-PF-co-Cz-PF), were soluble in common organic solvents and used as the EL layer in double layer light-emitting diodes (LEDs) (ITO/PEDOT/polymer/Al). All polymers show photoluminescence around λmax=430 nm (exciting wavelength, 370 nm) and blue EL around λmax=426 nm. The current-voltage-luminance (I-V-L) characteristics of the polymers show turn-on voltages of 3.5-5.5 V which are lower than that of PF2/6. The maximum brightness and EL efficiency of the device with the configuration of ITO/PEDOT/polymer/Al were 3000 cd/m2 at 10 V and 2.13 cd/A at 10.6 mA/cm2, respectively, which are all higher than those of PF2/6.